Scalability - You can load balance multiple instances of your application behind front end server. This will allow you to handle more volume, and increase stability in the event one of your instances goes down.
Security - Apache, Tomcat, and Glassfish all support SSL, but if you decide to use Apache, most likely thats where you should configure it. If you want additional protection against attacks (DoS, XSS, SQL injection, etc.) you can install the mod_security web application firewall.
Additional Features - Apache has a bunch of nice modules available for URL rewriting, interfacing with other programming languages, authentication, and a ton of other stuff.
Clustering - By using Apache HTTP as a front end you can let Apache HTTP act as a front door to your content to multiple Apache Tomcat instances. If one of your Apache Tomcats fails, Apache HTTP ignores it and your Sysadmin can sleep through the nigh